1. Field of the Invention
The present invention relates to a paper feed apparatus for a shuttle printer and particularly to such a paper feed apparatus comprising a platen and second paper holding means located forwardly of the platen.
2. Description of the Prior Art
There is well known a dot printer in which printing styluses are driven against a sheet of paper to be printed in accordance with information of printing to form letters, symbols or others with plural dots. Such a dot printer has been utilized as an output device in various information instruments. There is also well known a dot-line printer derived from the above dot printer, in which a plurality of printing styluses are arranged at regular space intervals on a single line corresponding to one line of the sheet and reciprocated by wire pitches to effect the desired printing while feeding the sheet in the direction perpendicular to the direction of feed. The dot-line printer is advantageous in that printing can be carried out at extremely high speeds. Since the pivotable part on which said plurality of printing styluses are mounted in line and which is reciprocated between wire pitches is called a shuttle, the above type dot-line printer is known as a shuttle printer.
In such a shuttle printer, a print sheet must be fed on each of dot-line printings. To increase the quality of printed letters, one requires a paper feed system which is operable to accurately feed print sheets.
FIG. 1 shows the primary components of a paper feed apparatus for use in the prior art shuttle printer. Print sheets 10 are housed within the printer and each drawn along a guide plate 12 into paper feeding drive means 16 through a platen 14, the drive means including a paper feeding tractor. The print sheet 10 is moved to a printing station by the drive means 16. During the movement of the print sheet 10, it is held against the platen 14 by means of a paper holding spring 18 to provide a back-tension to the print sheet 10.
Opposed to the platen 14 is arranged shuttle 22 which reciprocally travels in a horizontal direction along the print sheet 10, holding a plurality of printing styluses 20 therein. The shuttle 22 can be oscillated a predetermined stroke by the drive of shuttle driving means 24, whereat the printing stylus 20 is impacted against the print sheet 10 to form the desired dot-letter or symbol thereon.
However, such a printer is disadvantageous in that the print sheet 10 is slacked when the printing operation is once stopped. The slacking decreases the quality of printed letters when the printing operation is re-started.
More particularly, the shuttle 22 cannot rapidly be stopped on receipt of the instruction and oscillates for only a slight time until its motion is completed. When another instruction is provided to the printer, the oscillation of the shuttle 22 is re-initiated and continued until the speed thereof reaches a predetermined value only at which the printing stylus 20 is impacted against the print sheet 10 to form a dot-letter thereon.
At this time, as previously described, the back-tension is exerted to the print sheet 10 primarily by the fact that it is pressed against the platen 14 by the sheet holding spring 18. Therefore, due to vibration created by the oscillation of the shuttle 22 when the printing operation is stopped and then re-started, the biasing force of the spring 18 will be reduced to provide a slack in the print sheet 10.
If the printing operation is re-initiated when the print sheet 10 has such a slack, initially printed dots are overlapped by later printed dots, as shown in FIG. 2.
More particularly, when the print sheet 10 is to be moved through a predetermined distance after initial dots have been printed thereon, the movement of the print sheet 10 is used to accommodate the slack thereon between the platen 14 and the paper feeding drive means 16 so that the portion of the print sheet 10 engaging the platen 14 will not substantially be fed. As a result, new dots will be printed overlapping with the first printed dots until the slack on the sheet 10 is completely taken up.